The spinal column is formed from a number of bony vertebrae, which in their normal state are separated from each other by intervertebral discs. The intervertebral disc acts in the spine as a crucial stabilizer, and as a mechanism for force distribution between adjacent vertebral bodies. Without a competent disc, collapse of the intervertebral disc may occur, contributing to abnormal joint mechanics and premature development of degenerative and/or arthritic changes.
The normal intervertebral disc has an outer ligamentous ring called the annulus fibrosus surrounding the nucleus pulposus. The annulus fibrosus binds the adjacent vertebrae together and is constituted of collagen fibers that are attached to the vertebrae and cross each other so that half of the individual fibers will tighten as the vertebrae are rotated in either direction, thus resisting twisting or torsional motion. The nucleus pulposus is constituted of soft tissue, having about 85% water content, which moves about during bending from front to back and from side to side.
The aging process contributes to gradual changes in the intervertebral discs. The annulus fibrosus loses much of its flexibility and resilience, becoming more dense and solid in composition. The aging annulus fibrosus may also be marked by the appearance or propagation of cracks or fissures in the annular wall. Similarly, the nucleus desiccates, increasing viscosity and thus losing its fluidity. In combination, these features of the aged intervertebral discs result in less dynamic stress distribution because of the more viscousnucleus pulposus, and less ability to withstand localized stresses by the annulus fibrosus due to its desiccation, loss of flexibility and the presence of fissures. Fissures can also occur due to disease or other pathological conditions. Occasionally fissures may form rents through the annular wall. In these instances, the nucleus pulposus is urged outwardly from the disc space through a rent, often into the spinal column. Extruded nucleus pulposus can, and often does, mechanically press on the spinal cord or spinal nerve rootlet. This painful condition is clinically referred to as a ruptured or herniated disc.
In the event of the rupture of the annulus fibrosus, the subannular nucleus pulposus may migrate along the path of least resistance into the fissure forcing the fissure to open further. The increased size of the fissure can allow the migration of the nucleus pulposus through the wall of the disc. The migration of the nucleus pulposus may result in nerve compression and/or the introduction of inflammatory compounds into the space around the adjacent nerve roots. This compression and/or introduction of inflammatory compounds in the lumbar spine may adversely affect the nerves associated with the extremities, bladder, bowel and genitalia. The usual effect of nerve compression and/or inflammation in this region of the spine is intolerable back or neck pain, radiating into the extremities, with accompanying numbness, weakness, and in late stages, paralysis and muscle atrophy, and/or bladder and bowel incontinence. Additionally, injury, disease or other degenerative disorders may cause one or more of the intervertebral discs to shrink, collapse, deteriorate or become displaced, herniated, or otherwise damaged and compromised.
Surgical repairs or replacements of displaced or herniated discs are attempted approximately 390,000 times in the USA each year. Historically, there has been no known way to repair or reconstruct the annulus. Instead, surgical procedures to date are designed to relieve symptoms by removing unwanted disc fragments and relieving nerve compression. While results are currently acceptable, they are not optimal. Various authors report 3.1-21% recurrent disc herniation, representing a failure of the primary procedure and requiring re-operation for the same condition. An estimated 10% recurrence rate results in 39,000 re-operations in the United States each year.
Some have also suggested that the repair of a damaged intervertebral disc might include the augmentation of the nucleus pulposus, and various efforts at nucleus pulposus replacement have been reported. It is believed that nucleus replacement technologies may be enhanced through the use of complimentary annular repair. Furthermore, it is believed that various interbody technologies may be more readily sealed within the disc space through the repair of annular defects. Finally, annular repair may be utilized as a vehicle to deliver other reparative materials to the intervertebral disc space.